Ever since the discovery of carbon nanotubes, there has been great interest in the synthesis of 1-dimensional “nanostructures”. The rare earth elements are distinguished from other elements by their unique electron configuration which results in them having unique electrical, magnetic, optical and nuclear properties. Nanorods and nanotubes of rare earth oxides have great potential for use in catalyst and fuel cell applications, electronic and optoelectronic nanodevices. In particular cerium oxide and doped cerium oxide exhibits unique oxygen storage capacity, redox behaviour and ionic conductivity, the extent of which is likely to be dependent upon surface area and lattice structure. As such, considerable interest has surrounded efforts to produce high surface area cerium oxide based particles with unique morphology and high thermal stability.
Existing methods for the synthesis of 1-dimensional nanostructures described in the literature can be broadly classified in terms of methods that rely on vapour phase growth and methods that rely on solution based growth. Vapour phase growth typically relies on the evaporation of a suitable precursor material in a controlled atmosphere to produce elemental or oxide nanorods. Existing methods often require the use of a metallic nanoparticle catalyst. For example, Ge nanowires were grown over Au catalyst particles by the evaporation of Ge under vacuum [Wu Y. & Yang P. J. Am. Chem. Soc 123 (2001), p 3165-3166]; Si nanowires were generated by laser ablation and evaporation of Si and SiO2 under an argon atmosphere [Wang N. et al. Physical Review B 58 (1998), pR16024-16026]; and, ZnO nanowires were grown on Au coated Si substrates by heating a mixture of ZnO and graphite under an argon atmosphere [Huang MH. Et al. Advanced Materials 13 No2 (2001), p 113-116].
Solution based methods typically involve precipitation and growth from a solution with rod-like morphology being generated due to anisotropic growth dictated by crystal structure or through use of templates or capping agents which restrict growth in all but one direction. For example, it has been reported that the anisotropic nature of Te facilitated the formation of Te nanorods by the reduction of orthotelluric acid by hydrazine at various refluxing temperatures [Mayers B & Xia Y. J. Mater. Chem. 12 (2002) p 1875-1881]. ZnO rods were formed using a poly(vinyl pyrrolidone) capping agent during precipitation from a zinc acetate/propanol solution [Guo L. et al Materials Science & Engineering C 16 (2001), p 123-127].
Chemical synthesis of spherical and rod-like particulate rare earth compounds has been reported in the literature. Colloidal Gd, Eu, Tb, Sm and Ce(III) compounds were produced by aging rare earth salts at elevated temperatures in the presence of urea [Matijevic E & Hsu WP. J. Colloid and Interface Science 118 (1987) p 506-523]. The forced hydrolysis of Ce(IV) ions in the presence of sulphate ions in sealed tubes at 90° C. was reported to result in the formation of spherical or rod-like particles of hydrated cerium (IV) oxide [Hsu WP et al. Langmuir 4 (1988) p 31-37]. Rod-like particles of CeO2 were synthesised by a homogeneous precipitation method, whereby cerium nitrate solution was reacted with hexamethylenetetramine (HMT) and aged at 90° C. for 24 hours [Masakuni O et al. Journal of the Japan Society of Powder and Powder Metallurgy 50 (2003), p 354-358].
It has been reported that rare earth hydroxide nanowires can be synthesised by first dissolving Ln2O3 in concentrated HNO3 and then precipitating nanoparticles of Ln(OH)3 by adding KOH. Rod-like morphology was developed when the precipitate was subjected to subsequent hydrothermal treatment at 180° C. in a sealed autoclave for 12-24 hours [Wang X & Li Y. Angew. Chem. Int. Ed. 41 (2002), p 4790-4793]. In a related study, rare earth hydroxide nanotubes were synthesized by first dissolving Ln2O3 in dilute HNO3, then precipitating nanoparticles of Ln(OH)3 by the addition of KOH or NaOH. Tube-like morphology was developed when the precipitate was subjected to subsequent hydrothermal treatment at 120-140° C. in a sealed autoclave for 12-24 hours [Wang X & Li Y. Advanced Materials 15 (2003) p 1442-1446].
U.S. Pat. No. 3,024,199 teaches a process for producing stable hydrous rare earth oxide aquasols. An aqueous solution of at least one rare earth metal salt having monovalent anions is contacted with ammonia to produce a precipitate of the corresponding hydrous rare earth oxide. The precipitate is immediately washed to remove the ammonium salts, before peptizing the resulting washed hydrated hydrous rare earth oxide by heating at a temperature of about 60 to 100° C. for 5 to 60 minutes while agitating. The resultant sol contained rod-like particles.
The prior art methods and publications described above typically require the use of expensive starting materials, customised heat treatment facilities and/or specially controlled environments. There remains a need for an alternative method of producing rare earth nanorods. A process that can be conducted under ambient temperature and pressure conditions would be highly desirable.
It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms part of the common general knowledge in the art, in Australia or in any other country. In the statement of invention and description of the invention which follow, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.